The present invention relates to a non-aqueous electrolyte secondary battery and a process for the preparation thereof.
A battery which undergoes reversible reaction involving the absorption of lithium ion released from a positive electrode or a negative electrode by the other to perform charge and discharge has a high voltage and a high energy density and thus has heretofore been widely used as a power supply for consumers"" electronic devices. This type of a battery requires an electrolyte solution which cannot decompose within a wide potential range. Thus, a non-aqueous electrolyte solution has been used as such an electrolyte solution, and this type of a battery is referred to as xe2x80x9cnon-aqueous electrolyte secondary batteryxe2x80x9d (hereinafter also referred to as xe2x80x9cbatteryxe2x80x9d simply).
In particular, the lithium battery comprising a metallic lithium negative electrode which can provide a voltage as high as 3 V or more has been widely studied as a secondary battery which can achieve a high energy density. The metallic lithium negative electrode can be prepared, e.g., by pressure-bonding a metallic lithium foil onto both sides of a current collector of negative electrode made of copper foil or the like.
However, when repeatedly charged, such a battery can be subject to deposition of dendrite of metallic lithium on the surface of the negative electrode, causing shortcircuiting between the positive electrode and the negative electrode. Further, during discharge, dendrite is isolated from the negative electrode, occasionally causing the production of particles of electrochemically inert lithium. As a result, the charge and discharge efficiency is lowered, making it impossible to obtain a sufficient cycle life perforce.
The foregoing phenomenon is presumably attributed to the following mechanism. Metallic lithium can be oxidized very easily and thus can easily react with oxygen in the atmosphere during the preparation of the negative electrode to form an oxidized film on the surface thereof. Since this oxidized surface film has uneven in thickness, current is accumulated onto thin film area during charge and discharge, so that dendrite deposition is liable to occur.
In an attempt to enhance the charge and discharge efficiency for the purpose of solving these problems, hydrogen fluoride, (C2H5)4NF(HF)4 or the like has been used as an additive for electrolyte solution to cause metallic lithium to be deposited in spherical form instead of dendrite (xe2x80x9cJ. Electrochem. Soc.xe2x80x9d, 146 (1999) page 1693, xe2x80x9cJ. Fluorine Chem.xe2x80x9d 87 (1998) page 235, xe2x80x9cPreprint of 40th Seminary on Batteryxe2x80x9d, 1999, page 467). However, this attempt also cannot provide a sufficient cycle life performance. It is considered that the film on spherical metallic lithium particles deposited upon charge undergoes exfoliation due to change in the shape of metallic lithium during discharge, causing metallic lithium to be consumed for the formation of new film.
This type of a battery has another disadvantage. Since the oxidized surface film of lithium is inferior to metallic lithium in electronic conduction and adhesion, a sufficient electrical conduction cannot be attained, providing the battery with a raised internal resistance, even if a metallic lithium foil having an oxidized surface film formed thereof is pressure-bonded onto the current collector of negative electrode.
On the other hand, a so-called lithium ion battery comprising graphite or carbon instead of metallic lithium as a negative electrode and lithium cobalt oxide or lithium nickel oxide as a positive electrode has been conceived and has been used as a high energy density battery. Referring to this lithium ion battery, it is reported that the selection of a proper non-aqueous solvent makes it possible to cause reaction of the electrolyte solution with the carbon negative active material during initial charge, resulting in the formation of a film that acts as a protective film which inhibits further progress of the reaction. However, since lithium which has been consumed for the formation of the film cannot take part in charge and discharge, the negative electrode has a so-called irreversible capacity. It is thus reported that when the battery is overdischarged, the negative electrode loses its residual capacity before the positive electrode, causing the potential of the negative electrode to move to a drastically noble potential and hence causing the change of crystalline structure of the carbon negative electrode, which gives adverse effects on the subsequent battery performance.
In order to solve these problems, it has been proposed to laminate a metallic lithium foil on a positive electrode provided with a positive active material (Japanese Patent No. 3,030,995). In this report, in an attempt to inhibit the drastic rise in the negative electrode potential, the electricity to be supplemented upon initial charge by lithium laminated on the positive electrode is predetermined to be greater than the irreversible capacity of the carbon negative electrode so that the residual capacity of the positive electrode is lost before the negative electrode when the battery is overdischarged.
As another proposal, the use of Li1+xNiO2 (0 less than xxe2x89xa61) or Li1+yMn2O4 (0 less than yxe2x89xa61), which is obtained by adding excess lithium to LiNiO2 or LiMn2O4, makes it possible to make up for the irreversible capacity of the negative electrode and hence enhance the capacity of the battery (JP-A-9-306475 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d), JP-A-10-149828, JP-A-10-208730, WO9724773A1, J. Electrochem. Soc., Vol. 145, 1998, page 1131).
However, these lithium ion batteries are prepared by a process which comprises absorbing lithium ion by a carbon-based material as a negative electrode, they are inferior to batteries using a metallic lithium negative electrode with respect to energy density.
As another method for solving the problem of battery capacity drop due to irreversible capacity to enhance the energy density of battery there has been proposed a method which comprises laminating a metallic lithium foil on the negative active material. However, this method is also disadvantageous in that the resulting battery has a lower energy density than those using a metallic lithium negative electrode.
It is therefore an object of the invention to provide a non-aqueous electrolyte secondary battery which exhibits a reduced capacity drop, a reduced internal resistance of negative electrode and an excellent cycle life performance and to provide a process for the preparation thereof.
In accordance with the non-aqueous electrolyte secondary battery of the invention and the process for the preparation thereof, charging is carried out with a combination of a positive electrode provided with excess lithium and a negative electrode in order to cause lithium to be deposited on the negative electrode. Accordingly, no oxidized surface film is interposed between lithium and the current collector of negative electrode or the negative active material layer which is different from the case where a metallic lithium foil is laminated on the negative electrode. In this arrangement, a battery having a low internal resistance can be provided. Since the deposition of lithium is carried out in the assembled battery, lithium does not come in contact with air, preventing the formation of a thick uneven oxidized film on the surface thereof. Thus, the deposition of dendrite can be inhibited, making it possible to inhibit the drop of battery capacity and hence provide a battery having an excellent cycle life performance. Further, lithium can be retained on the negative electrode in excess to the capacity of the positive electrode. Accordingly, even when lithium is lost due to the deposition of dendrite or due to the reaction with the electrolyte solution, the drop of battery capacity can be inhibited because the negative electrode has excess lithium.